No changes, so why does it now run rich?

In early 2004 I had a conservative dyno tune with an autologic 4-bank chip with the intent of roadracing my S-Trimed 90 Mustang. After that initial tune the car ran great at idle, cruise and WOT. Made 450hp at a roadrace safe 11.6 air fuel. Life was good.

Throughout 2005 the WOT power felt like it was down particularly when the ambient air temps were in the 90s. So to check things out, we recently ran back to back runs on the dyno and the power is down to 350hp at somewhere below 10:1 AF. As soon as the throttle is mashed, the A/F drops below 10:1. Mind you the engine idles and cruises terrific at 14.7:1 or so while it was on the dyno. So it's got too much fuel -- but the only changes made to the car since the original tune in 2004 has been a tune up (same type of plugs, wires, cap, rotor, fuel filter, PCV valve) and a power pipe install the day before the most recent dyno run.

Before we start removing fuel from the chip I'd like to know what could have caused this drop in A/F. We don't think it's a bad meter (ProM 75mm Bullet) since it idles and cruises so well. Any ideas?

Setup:
306 shortblock
9.5:1 compression
Edelbrock Performer 5.0 Heads (1.94/1.60)
GT40 intake -- portmatched lower
65mm TB
75mm ProM plastic Bullet (for 42lb, supercharged, cone filter purchased new in 2004)
power pipe
B-303 cam
1.7 rockers
Vortech SQ-S Trim, stock pulleys, 12-13lbs with power pipe
42lb injectors
No FMU -- Chip tuned
Kirban regulator
41# fuel pressure with vacuum off
Walbro 255 high pressure in tank
T-Rex external
1 5/8 BBK longtubes -- port matched
2.5 Dr Gas X Pipe
2.5 Dynomax superturbo with turndowns